1. Field of the Invention
The present invention relates to a novel phopolymerization initiator useful for dental materials, photoresist materials, printing plate materials and hologram materials and, particularly, for dental materials. More specifically, the invention relates to a photopolymerization initiator which remains stable against the environmental light (having weak intensity) as compared to the conventional photopolymerization initiators but which quickly completes the polymerization in a very short period of time upon the irradiation with the intense light from such an irradiator as a halogen lamp, a xenon lamp or a laser diode, making it possible to obtain highly cured bodies, and without decreasing activity even after preserved for extended periods of time.
2. Description of the Related Art
There have been proposed a variety of photopolymerization initiators that generate radicals or ionic species upon the irradiation with the light to polymerize the polymerizable unsaturated compounds and cyclic compounds. In general, study has been forwarded concerning the photo-decomposition compounds that decompose upon absorbing the light to form polymerization activating species and systems of the photo-decomposition compounds combined with suitable photo-sensitizing agents, and such compounds have really been used.
As the photo-decomposition compounds, there have been known an acylphosphine oxide compound and an α-diketone compound. In particular, the α-diketone compound initiates polymerizing in the wavelength region of visible light which little affects the human body (e.g., a camphorquinone which is a representative α-diketone is a yellow compound having a maximum absorption wavelength of 468 nm). Further, a well-known combination of the photo-decomposition compound and the photo-sensitizing agent is the one of the α-diketone compound and a tertiary amine-compound. The above combination is useful in the field of dental materials since the α-diketone compound initiates polymerizing in the wavelength region of visible light.
In the field of dental materials, the above photopolymerization initiator is added to a paste-like composition (usually called composite resin) comprising chiefly a (meth)acrylate monomer and an inorganic filler to impart photopolymerizing property to the composite resin. The composite resin in the state of a paste is molded into the shape of a tooth and is cured by being irradiated with the light from a special light irradiator. Hereinafter, the light irradiated for curing by polymerization is often called “active light”. In general, the active light is emitted from a source of light of a luminous intensity of about 100 to 1500 mW/cm2 in a wavelength region of about 360 to 500 nm (main absorption region of the α-diketone compound) from a distance of about 0 to about 10 mm. In a dental clinic, for example, the composite resin blended with the photopolymerization initiator is filled in a cavity of a tooth to be restored and is molded in the form of the tooth, and is cured by polymerization by being irradiated with the active light by using a special light irradiator to thereby restore the tooth. Further, a dental technician applies the composite resin onto a plaster model in the form of a tooth that is to be restored, and cures it by polymerization by the irradiation with the light. Then, a dentist adheres the thus obtained cured body to the tooth by using a dental adhesive to restore the tooth (see, for example, “Basics of the Photopolymerizable Composite Resin and Clinics”, Hiroyasu Hosoda, Nihon Shika Shuppan Co., Feb. 10, 1986, pp. 9–20, Prior Art (A)).
However, when a combination of the α-diketone compound and the tertiary amine compound is used as the photopolymerization initiator, the viscosity of the composite resin (paste) increases while it is being filled or applied, making it difficult to carry out the operation.
That is, the operation for filling or applying the paste must be conducted under the illumination of incandescent light such as dental light for illuminating the oral cavity or room light such as fluorescent lamp (this kind of light is called environmental light) to make sure the shape of the paste and the color tone of the cured body obtained by polymerizing the paste. In general, the environmental light is adjusted to be about 500 to 10000 luxes for easy watching. The luminous intensity of environmental light over a range of 360 to 500 nm, which is a chief absorption region of the α-diketone compound is not larger than 1 mW/cm2 though it may vary depending upon the source of light, which is several percent of the active light at the greatest. However, the polymerization initiator of a combination of the α-diketone compound and the tertiary amine compound exhibits a favorable polymerization activity for the light of visible region. Due to its good polymerization activity, therefore, the polymerization initiator sensitively reacts even to the environmental light and initiates curing. Therefore, if the operation of filling or application is conducted under the illumination of environmental light, the high polymerization activity turns into a disadvantage; i.e., the curing of the polymerization initiator proceeds to arouse the problems described above.
The phenomenon of an increase in the viscosity of the paste during the filling or application operation can be avoided by decreasing the amount of addition of the photopolymerization initiator or by the addition of a polymerization inhibitor in slightly large amounts. When this method is applied, however, the curing does not take place to a sufficient degree despite of the irradiation with the active light for a period of time same as that of the prior art, arousing such problems that the cured body that is obtained exhibits a decreased strength and that unpolymerized monomer remains in large amounts near the surface of the cured body. To effect the curing by polymerization to a sufficient degree, therefore, the time for irradiation with the active light must be lengthened. In many cases, however, the above composite resin is used in the oral cavity of a patient, and lengthening the irradiation time not only prolongs the operation time but also causes an increased burden to the patient. It has, therefore, been desired to shorten the irradiation time (curing time).
Even with the composite resin (paste) featuring improved stability against the environmental light as a result of decreasing the amount of addition of the photopolymerization initiator, it is allowed to shorten the curing time or to increase the strength of the cured body by increasing the luminous intensity of the irradiated active light. However, an increase in the luminous intensity requires an increase in the amount of energy correspondingly. Besides, too intense light even though it is visible light causes disturbance to the human body and, particularly, to the eyes. In general, further, the source of light emitting a highly intense light also generates the heat in large amounts which may damage the human body (in recent years, it is a trend to lower the energy of the source of active light, and there have been widely used light irradiators employing a laser diode or the like to emit the light of an intensity of about 20 to 100 mW/cm2). That is, with the method of decreasing the amount of addition of the photopolymerization initiator, it is not allowed to shorten the curing time or to increase the strength of the cured body when there is used the light irradiator such as the laser diode, and it is difficult to effect the curing by polymerization quickly and to a sufficient degree without giving burden to the patient.
With the composite resin blended with the conventional photopolymerization initiator, as described above, it is not possible to enhance the stability against the environmental light without impairing the reaction activity for the active light. Namely, there has not yet been provided a composite resin having such properties that the curing does not take place with the weak light such as the environmental light and that the curing quickly takes place when it is irradiated with an intense light by using a dental irradiator.
In order to solve the above problems, a variety of photopolymerization initiators have been studied in addition to the combination of the α-diketone compound and the tertiary amine compound, as represented by, for example, a photocurable dental material comprising a (meth)acrylate polymerizable monomer, an acylphosphine oxide polymerization initiator and an amine compound (e.g., Japanese Unexamined Patent Publication (Kokai) No. 2000-16910 (prior art B)).
This dental material has a sufficiently high stability against the environmental light and exhibits high cured body properties, but requires the irradiation time for curing comparable to that of the conventional photocurable dental materials, and does not still satisfy the requirement of shortening the curing time.
It has further been known to use an aryl iodonium salt, a photo-sensitizing agent and an electron donor as polymerization initiator components (see, for example, U.S. Pat. No. 5,545,676 (prior art C)).
The composite resin containing the above polymerization initiator components can be cured by polymerization by being irradiated with the active light for a period of time that is shortened as compared to that of the conventional counterparts. However, the degree of shortening is not sufficient, and it has been desired to further shorten the time for irradiation with the active light (time for curing by polymerization). When the above photopolymerization initiator is used, further, there is seen no great improvement in the stability against the environment.
On the other hand, an s-triazine compound having a trihalomethyl group as a substituent is a compound that generates acid upon the irradiation with light, and has heretofore been used as a polymerization initiator for the photo-cationic polymerization. In recent years, however, the above s-triazine compound has been used as a radical polymerization initiator in combination with other components. For example, there has been proposed a radical polymerization initiator comprising a photo acid generator such as the above s-triazine compound, an aryl borate compound and a pigment that absorbs visible light (see, for example, U.S. Pat. Nos. 4,950,581 and 574,451 (prior arts D and E)).
In these photopolymerization initiators, the photo acid generator is decomposed by the irradiation with light to generate an acid which, then, decomposes the aryl borate compound to form active radical species that initiate the polymerization. The active radical species produced by the disintegration of the aryl borate compound offer such advantages as a very high polymerizing activity, are little impaired by oxygen from being polymerized as compared with the conventional radical polymerization initiators, cured within a short period of time, exhibit a sufficiently high activity even for the weak light, and are very useful as an adhesive for dental use.
Conversely, however, a sufficiently high activity for the weak light means that the stability is low against the environmental light. Therefore, it is not so desirable to use the above photopolymerization initiator for the dental composite resins.
It has further been proposed to use the s-triazine compound having the trihalomethyl group which is a substituent as the photopolymerization initiator in the addition polymerization compositions in combination with a photo-sensitizing compound and an electron donor compound (see, for example, EP 0369645 (prior art F)).
The above photopolymerization initiator has such advantages as a high polymerization activity and a high curing rate. According to the study conducted by the present inventors, however, the photopolymerizable composition blended with the above photopolymerization initiator exhibits a very high curing rate when there is used an irradiator equipped with a halogen lamp, but does not exhibit a favorable curing rate when there is used an irradiator equipped with a laser diode or a xenon lamp. Therefore, the irradiator is limited to the halogen lamp. Further, the above photopolymerizable composition has a defect in that the polymerization activity drops with the passage of time when it is preserved at a relatively high temperature of about 50° C. The dental materials are, in many cases, transported to dental clinics by using a passenger car. In the summer time, however, the temperature inside the vehicle often exceeds 50° C. Even if the temperature does not exceed 50° C., it is expected that the polymerization activity similarly drops when preserved for extended periods of time.